This chapter is a general introduction to the original turbo codes discovered in the early 1990s and known as convolutional turbo codes or parallel concatenated convolutional codes. It presents the main concepts of coding theory introduced with the invention of turbo codes, put in a historical perspective. The overall structures of the encoder and decoder are analyzed and some fundamental guidelines for the design of turbo codes with good performance are provided. Then, the basics of turbo decoding are introduced and the main component decoding algorithms are briefly described. Finally, the very first proof-of-concept implementations are presented and the pioneer telecommunication applications and current transmission standards using turbo codes are reviewed.